The invention relates to a thermal flow-rate sensor and a method for determining the flow rate of a fluid.
The practice of determining the flow rate of a fluid (i.e., a liquid or a gas) through, for example, a pipeline, by means of a thermal flow sensor is known in the art. Such a flow sensor comprises a heating device and a temperature detector which reacts to the temperature of the heating device. For the purpose of determining the flow rate, use is made of the cooling a effect on the flow sensor caused by the inflowing fluid. The greater the velocity of flow and, consequently, the flow rate of the fluid, the greater the quantity of heat that is taken away from the flow sensor by the fluid per unit of time through heat transfer and convection. Thus, for example, if a constant heating power is supplied to the heating device, the temperature detected by the temperature detector is lower in the case of a high flow rate than in the case of a low flow rate. Accordingly, if the heating power is regulated in such a way that the flow sensor is at a constant temperature, a higher heating power is required in the case of a greater flow rate than in the case of a lesser flow rate. Calibration is required in order to permit measurement of absolute flow quantities with such a flow sensor. This takes account, for example, of effects of the geometry of the flow sensor and the material properties of the fluid, which are substantial contributory factors in the determination of the heat dissipation capacity. Thus, for example, the heat dissipation capacity of the fluid depends on its composition.
Measurement of the flow rate by means of such thermal flow sensors is described in, for example, the textbooks of O. Fiedler, xe2x80x9cStrxc3x6mungs- und Durchfluxcex2mexcex2technikxe2x80x9d [xe2x80x9cFlow and flow-rate measuring systemsxe2x80x9d] (Oldenbourg-Verlag 1992), and H. Eckelmann, xe2x80x9cEinfxc3xchrung in die Strxc3x6mungsmexcex2technikxe2x80x9d [xe2x80x9cIntroduction to flow measuring techniquesxe2x80x9d] (Teubner-Verlag 1997).
In order to render possible the use of a flow sensor of the type outlined, the composition of the fluid whose flow rate is to be determined must not vary, or it must vary only within very narrow limits. This is because a different fluid composition could result in an alteration of the heat dissipation capacity, so that changes in the temperature or in the heating power of the flow sensor are not necessarily attributable to flow rate variations.
The object of the invention is to provide a thermal flow-rate sensor and a method for determining the flow rate of a fluid with the use of a thermal flow-rate sensor which also render possible flow-rate measurement on a fluid of variable composition.
This object is achieved by a thermal flow-rate sensor having the features of claim 1 and by a method, having the features of claim 14, for determining the flow rate of a fluid. Advantageous embodiments are disclosed by the dependent claims.
The thermal flow-rate sensor according to the invention comprises a flow sensor with a first heating device and a first temperature detector, reacting to the temperature of the first heating device, and can be exposed to a flowing fluid whose flow rate is to be determined. This flow sensor is a thermal flow sensor, based on the principle described above. In addition, the thermal flow-rate sensor has a thermal-conductivity measuring cell which comprises a measuring-cell casing, a second heating device and a second temperature detector, reacting to the temperature of the second heating device, the measuring-cell casing having at least one opening arranged for entry of the fluid into the measuring-cell casing.
A portion of the fluid can enter the thermal-conductivity measuring cell via the opening. Within the thermal-conductivity measuring cell, virtually no thermal transfer occurs through convection, since this measuring cell is designed for measuring the thermal conductivity of a fluid contained in it and its interior is therefore largely protected against disturbance by fluid flows. The thermal conductivity of the fluid can be determined by means of the second heating device and the second temperature detector, which reacts to the temperature of the second heating device. The fundamental measuring principle of the thermal-conductivity measuring cell is similar to that of the thermal flow sensor, but since no convection occurs within the measuring-cell casing, the heat removed from the second heating device by the fluid is transferred essentially through thermal conduction. Through selection of an appropriate geometry (particularly a small spacing between the second heating device and the inside of the measuring-cell casing) and an appropriate temperature range for the second heating device (relatively low temperatures are advantageous), the proportion of radiation in the heat transfer can be kept low. Such thermal-conductivity measuring cells, and the measuring principle used with them, are known in the art. The thermal conductivity of the fluid can therefore be determined by means of the thermal-conductivity measuring cell.
The thermal conductivity is a measure of the composition of the fluid. If the fluid has two components with different thermal conductivities, the proportions of the components in the fluid can be determined through a measurement of the thermal conductivity of the fluid. However, the thermal flow-rate sensor according to the invention can also be used, advantageously, for fluids with more than two components, as illustrated by the following example. If, for example, the fluid is a mixture of air and the hydrocarbons propane and butane, in respect of its thermal conductivity it consists essentially of two components, namely air and hydrocarbon. This is because, in comparison with air, propane and butane differ only slightly from one another in respect of their thermal conductivity properties and also have a similar heat dissipation capacity in respect of the flow sensor. In the case of this fluid, for example, the concentration of hydrocarbon in air can be determined, in the range from 0% to 100%, with an accuracy of better than 5%, by means of the thermal-conductivity measuring cell. The composition of the fluid is thus known with sufficient accuracy to enable it to be taken into account in the behaviour of the flow sensor.
The thermal-conductivity measuring cell thus makes it possible, by simple means, for the composition of the fluid to be determined or at least estimated to the extent that composition-dependent differences in the heat dissipation capacity of the fluid can be taken into account in the operation of the flow sensor in order to achieve a reliable flow-rate measurement. Since the thermal flow-rate sensor according to the invention comprises both the flow sensor and the thermal-conductivity measuring cell, it is ensured that all measurements are always performed on the same fluid.
The thermal flow-rate sensor preferably comprises a control and evaluation device which is arranged for the purpose of generating a first measuring signal, characterizing the heat dissipation capacity of the fluid, by means of the heating power supplied to the second heating device and the temperature of the second temperature detector, and generating a second measuring signal, characterizing the flow rate of the fluid, by means of the heating power supplied to the first heating device, the temperature of the first temperature detector and the first measuring signal. This first measuring signal is preferably assigned to the composition of the fluid. The control and evaluation device renders possible a preferably fully automatic operation of the flow sensor and thermal-conductivity measuring cell under the conditions explained above. In a preferred embodiment, the control and evaluation device is arranged for the purpose of determining the flow rate of the fluid by means of parameters, determined in calibration measurements, and the second measuring signal. By means of such calibration measurements, as already mentioned above, the quantities entering into the evaluation of the measurement results can be determined using predefined fluids in predefined conditions, so that it is possible, in principle, to output the flow-rate value of a flowing fluid directly from the control and evaluation device, for example on to a display or into a memory.
In a preferred embodiment, the thermal flow-rate sensor comprises a third temperature detector which is arranged for the purpose of measuring the ambient temperature. The accuracy of the thermal flow-rate sensor increases with the accuracy with which the ambient temperature is known. Thus, for example, the difference between the temperature of the second heating device and the temperature of the measuring-cell casing, which corresponds largely to the ambient temperature, enters into the determination of the thermal conductivity.
The control and evaluation device is preferably arranged for the purpose of regulating the heating power of the first heating device so that the temperature of the first temperature detector exceeds the ambient temperature by a predefined value. The control and evaluation device can also be arranged for the purpose of regulating the heating power of the second heating device so that the temperature of the second temperature detector exceeds the ambient temperature by a predefined value. Thus, for example, the temperature of the first heating device and the second heating device can be kept at 50 K above the ambient temperature through controlling of the respective heating powers. The instantaneous heating power is then a direct measure of the amount of heat removed from the flow sensor by the fluid per unit of time, or rather, of the amount of heat transferred away through thermal conduction per unit of time in the thermal-conductivity measuring cell.
In a preferred embodiment, the measuring-cell casing comprises a porous sintered body which is permeable to the fluid. The sintered body thus forms many small openings through which the fluid, from a region in which it may be flowing at high velocity, can enter the interior of the measuring-cell casing where it must not flow, since otherwise the measurement of the thermal conductivity would be falsified by a convection component. The measuring-cell casing preferably comprises a further opening at a distance from the sintered body. The measuring-cell casing can be of a cylindrical basic shape, the sintered body being disposed at an end, the further opening on the circumferential surface of the basic cylindrical shape and the second heating device and the second temperature detector in the region around the longitudinal axis of the basic cylindrical shape. In the case of such an arrangement, the fluid flowing past the further opening, outside the measuring-cell casing, produces within the measuring-cell casing a negative pressure which draws fluid into the measuring-cell casing through the sintered body. This ensures that the fluid is continuously replaced from the measuring-cell casing without disturbing convection movements and therefore corresponds, in respect of its composition, to the fluid flowing past the flow sensor.
The first heating device and the first temperature detector are preferably disposed, in thermal contact, in a first capsule; the same can also apply to the second heating device and the second temperature detector. In the case of this design, the first or second temperature detector directly assumes the temperature of the respective heating device, increasing the measuring accuracy.
In the case of a preferred embodiment, the thermal-conductivity measuring cell is disposed in the flow shadow, for example, in the flow shadow of the flow sensor or in the flow shadow of the third temperature detector. Contamination of the sintered body by dirt particles carried along in the fluid flow can be largely prevented through such an arrangement.
The first heating device and/or the second heating device can comprise a heating resistor which has a positive temperature coefficient in the operating range. Such a PTC heating resistance serves as a reliable safeguard against overheating, which is important, for example, if the fluid is an explosive gas. If the temperature of the heating resistor were to increase due to a fault in the open-loop or closed-loop control of the heating power supplied to the respective heating resistor, the resultant higher resistance value, at an upper voltage limit defined by the power supply, would cause a decrease in the heating current, so that the temperature drops. The system is thus inherently stable without additional safety devices.
The flow sensor and the thermal-conductivity measuring cell are preferably mounted on a common support and can be brought into contact with the fluid via a T-piece provided on a pipeline for the flowing fluid. Such a design provides for a clear overview, is low-cost and enables maintenance work to be performed easily.